Phosphors find applications in many Light Emitting Devices (“LED”). Thin films of phosphors are used in many imaging and LED applications from radiation detection to solid state lighting. The key properties of phosphors include quantum yield, stability and lifetime. In particular for LEDs the efficiency of conversion of high energy blue excitation light to the white light is a key factor for the overall LED efficiency. Phosphors are an integral part of any LED, and unfortunately contribute significantly to efficiency losses. The loss mechanisms include fundamental losses innate to the phosphor conversion material (nonradiative decay paths that lead to reduced quantum yields) and reduced extraction efficiency. The reductions in extraction efficiency include radial emission from the phosphor and wave guiding at interfaces. Phosphors are often applied in an epoxy layer over the high energy emitting GaN light source. If smooth layers are applied, effective wave guiding can occur that channels light to the sides of the device. Efforts have been made to reduce this effect through surface roughening but extraction efficiencies remain at or near 60% in the best cases. Typically phosphors are only available as powders or as thin film coatings.
One of the major obstacles in the development of high efficiency systems is loss due to wave guiding when thin film phosphors are used. Thin films help to minimize losses from self absorption, but the planar interface between the phosphor layer and other layers in the device lead to interfaces with different refractive indexes. At these interfaces all light from the phosphor that hits the interface at an angle greater than the critical angle as defined by Snell's Law is effectively reflected at the surface and wave guided to the edges of the film. One way to avoid this problem is to place the phosphor as a thin film on a three dimensional structure with vertical structures that allow the light to propagate in the desired direction. As the surface area of the 3-dimensional structure increases, more phosphor can be excited resulting in higher light yield. Porous structures offer great potential, but they are very difficult to coat. Two potential substrates include porous anodiscs that consist of a honeycomb structure with straight channels having pore diameters from 20 to 200 nm, and a second structure is posed on porous inverse opal structures having well defined connected cavities that can be readily controlled to the hundreds of nanometers, up to 500 nm. Both of these structures have high surface areas but the nanometer scale porosity with openings or cavities less than 900 nm make them very difficult to coat by traditional line-of-site techniques. More complex structures include mesoporous silica, such as Mobile Crystalline Materials (“MCMs”) which possess to some degree of ordered arrays of non intersecting hexagonal channels with the pore diameter of these materials within mesoporous range between 1 to 20 nm. Porous structures of this invention, therefore include pores in a range of from 1 to 500 nm.
Over the past 10 years, photonic crystal (“PC”) structures have emerged as perhaps the ultimate platform for microdevices that can manipulate light in all three dimensions. These artificial microstructures consist of a periodic repetition of dielectric elements, which creates forbidden and allowed energy bands for photons. PCs represent a major new frontier in optoelectronics due to their ability to coherently manipulate light. This manipulation is essential for enabling new concepts such as producing negative indices of refraction, tailoring the photonic density of states, controlling spontaneous emission rates, and modifying and controlling black-body radiation. It has been predicted [Shanhui Fan, et al., Phys. Rev. Lett. 78, 3294 (1997)] that a weakly penetrating etched photonic lattice on the surface of an LED can suppress all lateral modes, causing the light to be emitted primarily in the vertical direction.
Photonic crystal (“PC”) structures have emerged as perhaps the ultimate platform for micro-devices that can manipulate light in all three dimensions. PCs represent a major new frontier for a diverse set of properties including their ability to coherently manipulate light. It has been predicted that a weakly penetrating etched photonic lattice on the surface of an LED can suppress all lateral modes, causing the light to be emitted primarily in the vertical direction. PCs have been restricted to a subset of materials that can be formed in the sol-gel processing. It is not possible to make PCs from just any material, which limits their potential properties. Coating is one way to add functionality, but traditional techniques Pulsed laser deposition (“PLD”) and Chemical Vapor Deposition (“CVD”) cannot coat the complex porous structures. Sol-gel can penetrate the pores but does not result in conformal coatings since metal oxide oligomers form in the bulk solution. The primary technique used for effective coating of 3-D materials such as inverse opal structures is Atomic Layer Deposition (“ALD”). ALD is limited in that thicker coatings require many steps and only single component coatings can be readily applied. Polymer-Assisted Deposition (“PAD”) can deposit conformal coatings of complex metal oxides on nano-structured 3-D supports. This ability to form conformal coatings has led to the formation of completely new compositions of coated mesoporous silicon (silica) that emit light. Light emission from mesoporous silicon has been reported previously but never from conformally-coated materials.
A scintillator is a material that is transparent in the scintillation or emission wavelength range and that responds to incident radiation by emitting a light pulse. From such materials, generally single crystals, it is possible to manufacture detectors in which the light emitted by the crystal that the detector comprises is coupled to a light-detection means and produces an electrical signal proportional to the number of light pulses received and to their intensity. Such detectors are used especially in industry for thickness or weight measurements and in the fields of nuclear medicine, physics, chemistry and oil exploration. A family of known scintillator crystals widely used is of the thallium-doped sodium iodide Tl:NaI type. This scintillating material, discovered in 1948 by Robert Hofstadter and which forms the basis of modern scintillators, still remains the predominant material in this field in spite of almost 50 years of research on other materials. However, these crystals have a scintillation decay which is not very fast. A material that is also used is CsI that, depending on the applications, may be used pure or doped either with thallium (“Tl”) or with sodium (“Na”). One family of scintillator crystals that has undergone considerable development is of the bismuth germanate (“BGO”) type. The crystals of the BGO family have high decay time constants, which limit the use of these crystals to low count rates. A more recent family of scintillator crystals was developed in the 1990s and is of the cerium-activated lutetium oxyorthosilicate Ce:LSO type. However these crystals are very heterogeneous and have very high melting points (about 2200 degrees Celsius). The development of new scintillating materials for improved performance is the subject of many studies. One of the parameters that it is desired to improve is the energy resolution. This is because in the majority of nuclear detector applications, good energy resolution is desired. The energy resolution of a nuclear radiation detector actually determines its ability to separate radiation energies which are very close. It is usually determined for a given detector at a given energy, such as the width at mid-height of the peak in question on an energy spectrum obtained from this detector, in relation to the energy at the centroid of the peak. The smaller the value of the energy resolution, the better the quality of the detector.
Nevertheless, lower values of resolution are of great benefit. For example, in the case of a detector used to analyze various radioactive isotopes, improved energy resolution enables improved discrimination of these isotopes. While thin film scintillators have limited utility in applications where energy resolution is needed in radiation detection, they have major applications in imaging systems such as X-ray imaging device.
X-ray imaging devices in which the scintillator for converting an X-ray into visible light, or the like, and the imaging devices for receiving the visible light, or the like, are used in combination and more particularly a resolution-variable X-ray imaging device whose resolution can be changed as occasion demands and an X-ray CT apparatus. As the X-ray imaging device for capturing an image by visualizing an X-ray, there are some devices that can sense directly an X-ray and others that can visualize an X-ray by using the scintillator and then capture an image by using the imaging device such as CCD, or the like. In this case high quantum yield and very short lifetimes are desirable.
Conventional neutron detectors typically include devices that operate as ionization chambers or proportional counters. Each of the available methods demonstrates different strengths, but all share the common goals of high neutron efficiency, minimum gamma-ray sensitivity or gamma/neutron discrimination. Other systems, including scintillators doped with 6Li, or 10B, have been examined with mixed results. One of the prime difficulties in these systems is the gamma-ray rejection characteristics of the system. In addition, many of the detector materials are air and water sensitive or the scintillators employ heavy elements that limit gamma-ray rejection or have slow response times thanks to the long relaxation times. Scintillators have the added complication that often single crystals are required to avoid light loss, making it difficult to add large amounts of boron or lithium to increase the neutron cross-section absorption. While there are obvious advantages to the use of solid-state neutron detectors, to date these are outweighed by their disadvantages. Coating scintillators onto three dimensional structures which can then be filled with neutron stooping material may provide a new class of neutron detector.
Yttrium orthovanadate (“YVO4”) is an excellent polarizer and laser host material in its single-crystal form. Europium doping of YVO4 results in a red phosphor used in cathode ray tubes and color television in its powdered form. Europium-doped YVO4 thin films have been prepared through a variety of deposition techniques such as sol-gel process, CVD, PLD and microwave-assisted chemical solution deposition. YVO4 films prepared with these methods suffer from lack of crystallographic orientation control and the incorporation of vanadium-poor or rich nonstoichiometric phases.
Preparation of thin film scintillators is a difficult process. Generally, scintillators have a complex chemical composition and many methods to prepare high quality thin films are based upon high vacuum techniques.
Chemical solution deposition techniques have been generally viewed as less capital intensive (see, Lange, “Chemical Solution Routes to Single-Crystal Thin Films”, Science, vol. 273, pp. 903-909, 1996 and Schwartz, “Chemical Solution Deposition of Perovskite Thin Films”, Chemistry of Materials, vol. 9, pp. 2325-2340, 1997). Also, chemical solution techniques are not generally limited to flat surfaces.